Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: timbro52 on October 31, 2010, 02:45:11 PM
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I've just completed installation of the C4S upgrade on my SEX amp. Resistance values check out fine but in doing the voltage checks I'm getting 330 VAC on A5 B5 & A6 B6 and they should be 72 & 2.5 VDC respectively. Can anyone help me out. I've double checked my soldering and diodes are lighting up fine.
Thanks,
Tim
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You know, I've been looking over the schematic trying to figure out how that is possible.
How is it that you decided to measure AC Voltage on those particular terminals?
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Ok, let's think about this for a minute. The C4S shouldn't affect the power supply rectifiers. You built stock and the diodes and capacitors shouldn't have changed. So the voltage feeding the C4S should be DC already, not AC. It could be that your meter swaps between AC and DC automatically and some noise is confusing it into thinking it is AC.
Having high voltage on tube pins 5 and 6 indicates that there is no conduction in the cathode leg of the first triode. As I am not familiar with how the C4S is installed in the SEX amp I can't be too specific. I would guess that there is on C4S for the first plate and one for the first cathode. I would look at A6 and B6 to ground. See what is there and inspect it. If it is a C4S then are all the LEDs lighted?
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FYI, Grainger, there is only a single C4S on each side on the SEX.
I suspect that we are thinking along similar lines: There is some sort of issue with the reading which needs to be sorted out first. Hence, my question. I have been unable to come up with a scenario which would account for raw AC getting to those points.
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JC,
Thanks for the information. I don't have any information on the SEX other than a manual.
I will try to remember it but I can't remember yesterday so it is doubtful.
Does that mean it has one of the "universal" boards?
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It uses the original C4S board, one per side. Recently, I see that Doc upgraded that to a new "green" version PCB, with mounting hardware.
The original version (like mine), you'll recall, was basically held in place by its leads. Which worked fine in that it is a small, light board.
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Sorry guys, that should be 330 VDC not AC. Yes all the diodes are lighting up.
Thanks ,
Tim
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OK, so on each tube socket, Pins 5 and 6 measure 330 Vdc with respect to ground? And, the two LEDs on each C4S are lit? And, the only tube pin you worked on during the installation was Pin 5?
Have I got all of that straight? If so, I would double check that each lead on the C4S is going where it is supposed to, and that each transistor is correctly oriented.
I must admit, though, I am still stumped about how that Voltage can be getting to Pin 6 on each tube.
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Ok, now if pin 6 is at 330 volts, and is also 1270 ohms to ground, then the current through that 1270 ohm cathode resistor must be 260mA and it is dissipating 86 watts. It's rated 1/4 watt, so that seems unlikely :^) I conclude at least one of the premises is not true. The measurements may have seemed correct - probably because there is an intermittent connection somewhere, which changes with the mechanical forces of the measurement probe.
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Guys I think I found the problem and I'm more than a little embarrassed. I had removed the wrong resistors and soldered the jumpers to the wrong pins. (Oh, the learning curve of a newbie)
I was on A6 and B6 instead of A5 and B5. In partial defense, the instructions I believe are incorrect. It says, "Remove the 150K ohm plate load resistors from terminals 17 to A5 and 37 to B5.
After looking at the original S.E.X. amp instructions those resistors go from terminal 16 to A5 and 36 to B5. In hindsight I should have measured the damn things before I removed them.
So I'll wait patiently for Eileen to send me 2-1.27K 1/4 watt carbon resistors and hopefully that will do the trick.
Thanks for all the help!
Humbly Yours,
Tim
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Guys I think I found the problem and I'm more than a little embarrassed. I had removed the wrong resistors and soldered the jumpers to the wrong pins. (Oh, the learning curve of a newbie)
I was on A6 and B6 instead of A5 and B5. In partial defense, the instructions I believe are incorrect. It says, "Remove the 150K ohm plate load resistors from terminals 17 to A5 and 37 to B5.
After looking at the original S.E.X. amp instructions those resistors go from terminal 16 to A5 and 36 to B5. In hindsight I should have measured the damn things before I removed them.
So I'll wait patiently for Eileen to send me 2-1.27K 1/4 watt carbon resistors and hopefully that will do the trick.
Thanks for all the help!
Humbly Yours,
Tim
Nothing to be embarrassed about - for those of us who aren't electrical engineers (I myself am a psychologist, for heaven's sake), making mistakes seems to be the best way to really learn about how things work. Looks like you removed the cathode resistors for the small triode halves of the tubes vs. the plate loads. If you have any HLMP-6000 LEDs laying around, you can use one in series with a 500 ohm resistor, or two in series, in place of the 1.27k resistors. This may actually give a cleaner sound than the 1.27k resistors, though I myself seem to prefer the resistors over the LEDs in that particular instance, having tried both ways. If not, Eileen is pretty quick with shipping!
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No need for embarrassment whatsoever. One of the trickier things in electronics is to keep your place when the pins are in a circle, IMO. Every time I turn a chassis over to look at a tube socket, I take several minutes to re-orient myself; that's how many times I've counted the pins from the wrong end of the circle.
Be sure to post back when you take your readings after the re-work; I always look forward to hearing how these things turn out!
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I built my first kit in 1964. I number as many pins as it takes to keep my place. Numbering is not intuitive.
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Well I'm back needing assistance again. I replaced the the 2-1.27K resistors in the right locations this time and when I went to fire it up the fuse blew. I've been over the manual a zillion times and for the life of me I can't find my mistake. Resistance values are all OK, checked and rechecked my solder joints but still when I power up the fuse blows. Can anyone point me in the right direction.
Thanks,
Tim
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I wonder if the tubes themselves may have been damaged. I would suggest trying again with the tubes removed, to see if the fuse will hold.
If my guess is incorrect, of course, you're going to blow another fuse, I'm afraid. Sorry about that.
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I'm sure that you have looked at it a million times but check the orientation of the power supply caps. If the fuse is blowing before the c4s leds light up there isn't a lot that will cause that other than something shorted between pins or backwards caps. If you can't find the problem try getting a can of air and blowing out the transformer. I have had wire clipping fall into a couple of power transformers that caused weeks of trouble shooting only to be solved with a can of air.
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If the fuse is blowing before the c4s leds light up there isn't a lot that will cause that other than something shorted between pins or backwards caps.
Or blown rectifier diodes - which usually blow because caps were put in backwards.
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Thanks Jim,
The fuse held, now what? Is it time to email Bottlehead for new tubes? Did I blow the tubes with my miss wire at the pins?
Humbly yours,
Tim
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Well, we still don't know exactly what's going on, we just know that something is drawing too much current when the tubes are in the amp and the circuits are complete.
My next step, since I have a tube tester, would be to test the tubes. If you don't have one available, I always recommend looking in the phone book to see if there is a guitar shop in your area that repairs tube guitar amps. They usually have a tester of some kind.
In the meantime, since you can now power up at least part of the amp, it may be useful to try to get some Voltage readings. Expect them to be mostly higher than in the manual, since the tubes are out.
A good place to start, I think, would be the tubes socket pins on each socket. Post those back here. Note: IIRC, the readings should all be DC and with respect to circuit ground.
BTW, even though the fuse is holding, I would not recommend leaving the amp "On" any longer than it takes to get a few quick readings. Doing so can tend to lead to some complacence, and even without the tubes in place, there is still ample opportunity for a nasty shock!
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Try one tube, if the fuse holds, move it to the other side. then try the other tube...John